If my child was extremely vulnerable, I would go exploring for quieter playgrounds or quieter time slots. I would also be weighing up the likely risks of transmission in my area, the nature of my child's condition and the long term developmental costs of physical and social isolation. The conclusion would depend on my child's needs and the risks in the area. There is not a one-size-fits-all answer.
As it is having children of normal health, one of which with ASD, their needs to play naturally outside the home after 4 and a half months of being denied normal social (and edicational) opportunities means that the risks of catching Covid in an outdoor environment from fleeting contact with other children are miniscule and far outweighed by the social and physical benefits of normal play.
My 9yo has ASD and can be reticent about unknown people. I am not going to encourage him to be tormented by social phobias for the rest of his life by teaching him to avoid other people incase they are disease ridden and then have to reteach normal social behaviour. Back in June, my normally sociable, energetic 7yo was getting dangerously close to depression, crying that he needed new friends because he felt abandoned after months of isolation, and spent days of grotty weather flopping lethargically on my bed watching TV vacantly. That was the point that I decided that any playground that could be entered without damage was fair game.
The reality is that children don't tend to naturally get right into each other's faces. On equipment like slides or swings, they tend to face the same direction which is lower risk. The strong UV at this time of the year also rapidly weakens and destroys the virus. That's assuming that one of the children is 1:**,000 of infected people out in society without symptoms anyway. If I was that concerned about those odds, I'd be buying a lottery ticket.